Is it to much light for a single mushroom? yes. Is it to much light for some SPS Corals, no. Just when you switch over you need to be careful and place a screen over the tank so your corals don't get hit with all that light at once. You could also keep the fixture up higher over the tank say around a foot or more above the top of the water and/or also reduce the amount of time the lights remain on during the day,
A friend of mine as a 180g tank with 4 x 400W MH with 2 (i think) 110W VHO above his tank and its doing great.
Is your current lighting not working?